The present invention relates generally to metrology of semiconductor manufacturing processes. More particularly, the present invention is a needle comb reticle pattern for simultaneously making critical dimension (CD) measurements of device features and registration measurements of mask overlays relative to semiconductor wafers during processing of semiconductor wafers.
Lithographic and etch processes used in semiconductor device manufacturing are subject to variations in exposure, focus and alignment. Generally, less variation in such semiconductor manufacturing processes leads to higher yields. Because of such process variations, patterns developed by lithographic processes must be continually monitored, or measured, to determine if the dimensions of the patterns are within acceptable ranges and to determine whether the multiple mask layers, or overlays, are properly aligned and not biased or skewed.
Conventionally, monitoring of pattern features and measurement of critical dimension (CD) may be performed using a scanning electron microscope (SEM) tool and/or an automatic force microscopy (AFM) tool. Pattern placement is measured with an optical registration tool. While SEM metrology provides very high resolution, it is expensive to implement and relatively slow in operation.
AFM also provides high resolution but is typically even slower than SEM. The terms xe2x80x9coptical metrology system,xe2x80x9d xe2x80x9coverlay metrology systemxe2x80x9d and xe2x80x9cregistration toolxe2x80x9d are used interchangeably herein. Optical metrology systems overcome many of the problems associated with SEM and AFM metrology but are unable to resolve adequately for feature dimensions of less than about 1 xcexcm. SEM measurements can take 8-15 seconds per measurement depending on the complexity of the task being measured, e.g., number of scans per wafer, location of measurement points, etc. AFM measurements are measured in minutes per measurement (not seconds) because of long setup times and slow scan times. A registration tool can make measurements approximately every 0.5 to 2 seconds, or at a rate of approximately 1800 to 7200 measurements per hour. Thus, registration tools are clearly faster than SEM or AFM and, hence, more suited to in-line process measurement.
State-of-the-art semiconductor devices have submicron features. Accordingly, there is a need in the art for systems and methods for monitoring pattern features with dimensions on the submicron level that are inexpensive, capable of high-speed operation and subject to automation.
U.S. Pat. No. 5,701,013 to Hsia et al. discloses a wafer metrology pattern integrating both overlay and critical dimension features for SEM and AMF measurements. The Hsia et al. pattern, or test mask target, contains lines measuring 0.25 xcexcm, 0.3 xcexcm and 0.5 xcexcm in width as well as a centrally positioned box for reference. The Hsia et al. pattern is intended to be measured with a metrology tool utilizing AFM.
U.S. Pat. Nos. 5,712,707 and 5,757,507, both to Ausschnitt et al., disclose a wafer target for determining bias or overlay error in a substrate formed by a lithographic process and methods for using same. The ""707 and ""507 Ausschnitt et al. target includes a pair of straight vernier arrays of parallel elements, a staggered vernier array of parallel elements, and, optionally, at least one image-shortening array on the substrate. The ""707 and ""507 Ausschnitt et al. target allows measurement of bias and overlay error in deposited lithographic etch patterns that are human readable during substrate processing.
U.S. Pat. No. 5,805,290 to Ausschnitt et al. discloses a level-specific target array and method of optical metrology of unresolved pattern arrays. The ""290 Ausschnitt et al. level-specific target array includes a first target portion with four outer arrays of parallel lines oriented coincident with the sides of a box and a second target portion with four inner arrays of discrete, square elements oriented within and concentric to the outer arrays. First and second target portions are printed on different levels during semiconductor fabrication and allow for measurement for bias and overlay error with conventional optical metrology tools and without use of SEM or AFM tools, except for calibration.
U.S. Pat. Nos. 5,953,128 and 5,976,740, both to Ausschnitt et al., disclose optically measurable, serpentine edged, reverse tone targets used in controlling focus and exposure parameters of lithographic processes and methods for using same. The ""128 and ""740 Ausschnitt et al. targets also disclose needle comb elements with symmetrical stepped tapering and single-sided stepped tapering. The invention provides complementary tone patterns of shapes and spaces on a resist film on a substrate. Bias and overlay errors are then measured as functions of deviations from the expected lithographic etching norms provided by the reverse patterns.
U.S. Pat. No. 6,023,338 to Bareket discloses a target, associated apparatus and methods for determining offset between adjacent layers of a semiconductor device. The Bareket target is composed of alternating periodic structures on two successive layers of a semiconductor wafer. As electron beams are scanned across the periodic gratings of the Bareket target, relative phase shift between the two layers, and hence alignment, may be determined.
Ausschnitt et al., Seeing the Forest for the Trees: a New Approach to CD Control, SPIE Vol. 3332, 1998, pp. 212-20, discloses optical critical dimension measurement of pattern arrays (also known as Schnitzl arrays) whose individual features need not be resolved by the metrology tool. However, Schnitzl arrays alone do not provide for registration measurements to detect bias and overlay error.
Kim et al., Automatic In-situ Focus Monitor Using Line Shortening Effect, SPIE Conference on Metrology, Inspection, and Process Control for Microlithography XIII, Santa Clara, Calif. , March 1999, pp. 184-93, discloses a box-in-box pattern with conventional line and space patterns along edges of the inner and outer boxes. Kim et al. considered dagger (or wedge) tapered needle points to be the most sensitive to the line shortening effect. However, Kim et al. concluded that dagger needle points are impractical because of limitations in the angles allowed with most electron-beam mask generation systems, e.g., 0xc2x0, 45xc2x0 and 90xc2x0. Kim et al. also contemplated the use of stair-stepped needle patterns but rejected them because of the prohibitively large electron-beam mask data size and difficulty in checking and verifying defect-free masks with complex stair-stepped needle patterns.
While these prior art approaches have addressed many problems faced by semiconductor process control engineers, there still exists a need in the art for a target or pattern that simultaneously provides for critical dimension analysis and registration measurements, that is cost effective, capable of high-speed operation, subject to in-line process automation and that can be placed in scribe lines at arbitrary locations on a semiconductor wafer.
The present invention comprises needle comb reticle patterns that allow registration measurements and CD measurements using a single registration tool at a single location on a semiconductor wafer. The needle comb reticle patterns of the present invention save time and cost by eliminating the need to make registration and CD measurements separately with different tools.
An embodiment of a needle comb reticle pattern of the present invention includes a box-in-box registration feature, surrounded on two adjacent sides by needle comb features. Another embodiment of a needle comb reticle pattern of the present invention includes a box-in-box registration feature, surrounded on two adjacent sides by needle comb features and reference bars adjacent to the other two sides of the box-in-box feature. Yet another embodiment of a needle comb reticle pattern of the present invention includes a box-in-box registration feature surrounded on all sides by needle comb features. A preferred embodiment of a needle comb reticle pattern of the present invention includes two complementary needle comb reticle subpatterns. Needle comb features on opposite sides of the box-in-box registration feature have needles pointing in the same direction.
The box-in-box feature includes an inner box and an outer box which are mutually concentric. Each needle comb feature includes a base from which stair-stepped, tapered needles extend and point toward the outer box of the box-in-box feature. These stair-stepped needles have a variable width, much smaller than length, i.e., variable width less than  less than length. The variable width of each stair-stepped needle runs from larger-than-stepper resolution to subresolution width. When viewed with an optical tool, the comb feature will appear as an opaque bar of a width that depends on the degree to which the stepper system is in focus. The more out of focus, the narrower the width of the comb feature. The individual needles cannot ordinarily be resolved without the aid of high magnification, such as may be provided with a SEM tool.
The needle comb reticle pattern of the present invention allows use of a single registration tool to measure both wafer alignment and resolution, as characterized to critical dimension (CD), saving time ordinarily devoted to separate measurements. Furthermore, the needle comb reticle pattern may be placed anywhere on a semiconductor wafer in the scribe lines, thus, conserving wafer real estate for production devices.
During the wafer alignment process, a needle comb reticle pattern is viewed with an optical measuring device, such as a registration tool. Suitable registration tools, also known as overlay metrology systems, include, e.g., model numbers 5200XP and 5300, from KLA-Tencor, San Jose, Calif. When viewed by the registration tool, the subresolution portion of the needles within the needle comb features will appear shortened. The extent of shortening depends on the lens of the stepper tool, its optical setup and process capability. The registration tool can then measure the apparent shift in the needle comb marks relative to the center of the overlay printed at the same level. This allows for an amplified measurement in relation to the exposure system""s resolution capability. Furthermore, CD and focus measurements may be taken while simultaneously measuring for registration. Using a registration tool in this way is generally faster than previous registration tools used in conjunction with SEM or AFM tools.
Reticles, fields of reticles, masks and wafers including the needle comb reticle pattern of the present invention are encompassed by the present invention. Additionally, methods of analyzing critical dimension and registration of microelectronic lithographic processes and for characterizing measurements from a needle comb reticle pattern to pattern critical dimension are also encompassed by the present invention.
These embodiments, methods and attendant advantages of the present invention will be readily understood by reading the following detailed description in conjunction with the accompanying figures of the drawings.